1. Field of the Invention
The present invention relates to a light emitting display, and more particularly to a pixel circuit coupled to a plurality of organic light emitting diodes (OLED) that emit light so that it is possible to improve the aperture ratio of the light emitting display, to compensate for the threshold voltage, and to thus make brightness uniform and a light emitting display using the same.
2. Discussion of Related Art
Recently, various flat panel displays of lower weight and volume compared with cathode ray tubes (CRT) have been developed. In particular, light emitting displays having improved luminous efficiency, brightness, view angle, and high response speed are spotlighted.
In an OLED, an emission layer made of a thin film that emits light is positioned between a cathode electrode and an anode electrode. Electrons and holes are injected into the emission layer and are recombined to generate exciters at a reduced overall energy. Light is emitted as a result of this recombination.
The emission layer of the OLED may be formed from organic or inorganic material. The OLED is divided into organic and inorganic OLEDs according to the type of the emission layer.
FIG. 1 is a circuit diagram illustrating a part of a conventional light emitting display. Four adjacent pixels are shown that each include an OLED and a pixel circuit. The pixel circuit includes a first transistor T1, a second transistor T2, a third transistor T3, and a capacitor Cst. Each of the first, second, and third transistors T1, T2, T3 includes a gate, a source, and a drain and the capacitor Cst includes a first electrode and a second electrode.
Because the pixels all have the same circuit, only the left top pixel will be described. The source of the first transistor T1 is coupled to a power supply line Vdd, the drain of the first transistor T1 is coupled to the source of the third transistor T3, and the gate of the first transistor T1 is coupled to a first node A. The first node A is coupled to the drain of the second transistor T2. The source of the second transistor T2 is coupled to a data line D1, the drain of the second transistor T2 is coupled to the first node A, and the gate of the second transistor T2 is coupled to a first scan line S1. The second transistor T2 transmits a data signal to the first node A in response to the scan signal applied to its gate. The first transistor T1 supplies current corresponding to the data signal to the OLED.
The source of the third transistor T3 is coupled to the drain of the first transistor T1, the drain of the third transistor T3 is coupled to the anode electrode of the OLED, and the gate of the third transistor T3 is coupled to an emission control line E1 to respond to an emission control signal. Therefore, the third transistor T3 controls the flow of current that flows from the first transistor T1 to the OLED in response to the emission control signal to control emission of the OLED.
The first electrode of the capacitor Cst is coupled to the power supply line Vdd and the second electrode of the capacitor Cst is coupled to the first node A. The capacitor Cst is charged according to the data signal and applies a signal to the gate of the first transistor T1 during one frame and therefore maintains the first transistor T1 operating during the one frame.
However, in the conventional pixel circuit for the light emitting display, because each OLED is driven by a pixel circuit of its own, a plurality of pixel circuits are necessary to drive a plurality of OLEDs. This design feature increases the number of pixel circuits that form the display.
Also, because one emission control line and a pixel power supply line are coupled to each pixel row, wiring lines become complicated and the aperture ratio of the light emitting display deteriorates.
Therefore, there is a need for an alternative design that reduces the number of the pixel circuits and simplifies the arrangement of the wiring lines.